1. Field Of The Invention
The present invention lies in the field of optical imaging systems. More specifically the invention concerns magnifying systems like high resolution optical microscopes.
2. DESCRIPTION OF THE PRIOR ART
There exists a great variety of optical magnifying systems using classical optical elements like lenses, prisms and mirrors. The imaging characteristics can be designed for different purposes, e.g. including color filtering, polarizing or darkfield imaging. All these systems have in common that objects or details having a size close to or below the wavelength of illuminating light can't be resolved due to light diffraction phenomena. Thus standard microscopes, which use lenses for magnifying small objects, are limited in resolution due to wave characteristics of light. This limiting effect is expressed in the well known theorem of Abbe, which describes the maximum achievable lateral resolution of imaging systems as a function of the wavelength of the light and the aperture of the optical system.
Recently a new type of instrument was disclosed by D. W. Pohl, U. Ch. Fischer and U. T. Duerig, "Scanning near-field optical microscopy (SNOM): basic principles and some recent developments", Society of Photo-Optical Instrumentation Engineers, Scanning Microscopy Technologies and Applications, 1988, pp. 84-90, which was capable of resolving details below the wavelength of illuminating light, thus overcoming Abbe's criterion. The underlaying concept of this specific non imaging system is the illumination of an object through a pinhole having an aperture diameter of only a fraction of the light wavelength. This pinhole is placed next to the surface of an investigated object. Backscattered light from the object's surface can be detected through the pinhole. For very thin objects transmitted light can be detected alternatively from the backside of the object. A full picture of at least parts of the object is generated by scanning the pinhole over the object thus sequentially receiving intensity fluctuations of detected light due to variations of the object's reflectivity or transmittance respectively.
Although a very high resolution can be achieved with this pinhole system there are still some inherent restrictions to consider.
For maximum resolution the distance between pinhole and object surface should not exceed the pinhole diameter.
A considerable time is required to generate an image by mechanical scanning. Drift and distortion must be controlled.
Reflection objects must be illuminated and sensed by the same pinhole. The very small amount of light that finds its way back through the pinhole is overlaid by unwanted backside reflections.
Phase objects (e.g. surface micro-roughness) cannot be detected directly. Some depth information can only be derived from the signal intensity, that decreases as a function of the distance between pinhole and object surface.
It is the object of the present invention to disclose an improved device, that allows real-time imaging of surface contour and reflectance information with a lateral resolution that overcomes the Abbe diffraction limit.